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Why don't we see more bikes made with square tubes?(29 posts)

Why don't we see more bikes made with square tubes?niteschaos
Jan 21, 2004 9:44 PM
Or made of small composite I-beams? I was reading on the efficiency of structures and found that the beams which have the material farthest from the nuetral axis (the imaginary plane through the member where shear forces are highest and bending moments are lowest) are more effecient at carrying a load per unit mass. That explains why I-beams and box beams are used in structures from airplanes to sky scrapers, where you hardly see tubes. These shapes all have a higher section modulus than circular tubes, the shape of choice that most bikes are made of.

Why don't we see bikes made out of scaled (Al or composite) I-beams or even square tubes if the math (had to do it for homework so I'm pretty familiar with it now) shows that these cross sections produce higher section modulus per mass and would therefor allow us to have lighter bikes at that same strength and stiffness (do to the reduced amount of mass needed)?

This question is for all you engineers out there. I hope to join you all soon. I bet the answer lies in asthetics and manufacturing cost or something lame like that.
re: Why don't we see more bikes made with square tubes?Woof the dog
Jan 16, 2004 4:45 PM
aerodynamics most of all

besides, square tubed bikes are stupid.

you are stupid

... just kidding :0)

Actually, he is. (nm)Dave_Stohler
Jan 22, 2004 12:38 AM
Engineer's response:Dave_Stohler
Jan 22, 2004 12:24 AM
OK, where do I start?

Square tubes are frightfully non-aerodynamic.

Square tubes cost more to extrude. Forget even trying to butt them...

Square tubes are harder to weld to round objects, such as BB's, headtubes.

Square tubes are not as strong as round tubes. You'll learn this during next week's lecture in your "strength of materials" class.

I'm still trying to figure out how on earth you got the idea of a square tube having a higher strength modulus than a round tube. A circle is always stronger than a square. I doubt you're really doing well in your class. Please study harder. Don't apply to Pi Tau Sigma just yet.

P.S.-this is a good example as to why a little knowlege is a dangerous thing.
I know you're a troll but...amflyer
Jan 22, 2004 8:01 AM
...I am so tired of dealing with ass-bags like you. Higher education indeed.

Prove it.niteschaos
Jan 22, 2004 9:24 AM
Prove it. Let's see some numbers on how you got that a square tube of same mass per unit length is stronger than a circular tube? I wanna see your assumptions buddy. Oh, and I'm doing really well in that class.
Obviously, you are brain-deadDave_Stohler
Jan 22, 2004 7:51 PM
If you read my post, I did not say that a square tube was stronger. In fact, you, were the one who seemed to suggest that they were. Please go back, read my post again, and notice that I said that a round cross-section is always stronger than a square one. Then, when you sign up for next semester's classes, please take a course in reading comprehension.
Once you pull your foot out of your mouth, post another lame response.
I can hardly wait....
Do you eat with that mouth?niteschaos
Jan 23, 2004 4:37 AM
How do you think it is okay to just come off on a message board like a Dogmatist?
ah, the arrogance of youthmohair_chair
Jan 23, 2004 7:11 AM
Before you get too carried away basking in your own brilliance, remind yourself that there's a reason you are still in school. It's because you don't know it all. Even when you get out of school you'll quickly discover that the difference between what you learn in school and what you do in the real world is vast.

A bit of career advice for you: If you can't back up your arrogance with true brilliance, you better learn to temper it, otherwise you are going to get your ass handed to you in your first job. Eventually you'll find yourself putting all that engineering expertise into determining if a square wastebasket is better than a round one.
My karma ran over your dogma!Dave_Stohler
Jan 23, 2004 9:20 PM
And, BTW, I've been eating with this same mouth since long before you were even a zygote, you snivelling little git!

re: Why don't we see more bikes made with square tubes?yeah right
Jan 22, 2004 12:24 AM
I-beams might work great if the bike was only subjected to planar stresses on each tube, ie. you only bent each of the tubes in one direction. In this case, I-beams would certaintly stiffen the bike.

However, this is not the case. A circle has an even distribution of material, all at an equal distance away from the neutral axis. Therefore, for a generalized moment (or torque) a circle is your best cross-section.

Why not square tubes? Things always fail at corners. Why? Something called shear flow, which basically induces a localized stress when you try to twist a tube which has corners.

Oh and your I-beam bike would fold up if you even nicked the i-beam, because the wall thicknesses would be so thin to keep it light.

Five years of school, and $150k, and I can answer these things:) June and my masters degree can't come soon enough.
I agree with you guys on the I beams but not so surebimini
Jan 22, 2004 5:26 AM
about the square tubing. I don't have my steel book handy with all the facts and figures but I do recall I always liked working with square tubing on machines.

If I recall right a 2" x 2" square tube had better x, y strength than a 2" round tube of equal thickness, and torsional strength was about the same. There may be a rub when you compare strength/verse weight. A 2" square tube will weigh more than a round tube of equal thickness, so they may end up about equal or the round tube slightly ahead.

Stress concentration in corners is not a big issue since in standard retangular shapes the corners are not sharp. The inside and outside corners are radiused which minimizes or eliminates stress concentration.

In regards to square shapes being hard to weld. I totally disagree on this point. Rectangular shapes are easier to cut and clamp up than round tubing. You can clamp it up using standard C Clamps. Can't do that with a round tube.

Another nice thing about the rectangular shapes is if you have heavy loading in one axis and lighter loading in the other (maybe like in a bicycle) you could go with say a 3" x 2" rectangular and optomize the shape and weight to strength ratio where it is needed.

Is a square tube aerodynamic. Well No.

I have never seen tapered nor butted square tubing either, nor super thin wall tubing so it would be tough to build a frame out of materials that don't exist.

Probably the real reason you don't see the odd shapes much is tradition and customer acceptance. I've seen many excellent engineering designs flop when they hit the marketplace because they did not fit in with the customers concept of what the item should look like.

PS: The goverment spent a 100k to put me through school back in the 70s and I been designing and directing the develop of machines for another 25+ years.
oval tubeslaffeaux
Jan 22, 2004 9:58 AM
The '3" x 2" rectangular' tube that you contemplate has been used on occasion. Yeti (and I'm sure others) have used a top tube that was an oval (on some bike models). This gives more rigidity in one plane than the other.
Jan 22, 2004 9:26 AM
This was the answer I was looking for. Thanks for taking your time.
re: Why don't we see more bikes made with square tubes?Juanmoretime
Jan 22, 2004 2:45 AM
I can,t believe you guys are giving away Radialronpruitt's bike design ideas before he has them patended or brought them to the market.
re: Why don't we see more bikes made with square tubes?lithiapark
Jan 22, 2004 6:26 AM
Actually some square tubes are used. The lightspeed ad at the top of this forum page shows a top tube that is square, it is just turned with its diagonal in the plane of the frame, and called a diamond.
I think they would hurt more in a crash! nmdzrider
Jan 22, 2004 6:34 AM
They do....52-16SS
Jan 22, 2004 6:49 AM
Many frames have somewhat square profile chainstays (e.g. Columbus Squadra Course) so it's more a matter of what works best a given location.

I thought I-beams are directional and round omni directional nmcyclopathic
Jan 22, 2004 6:56 AM
More than engineering...spluti
Jan 22, 2004 7:43 AM
Go ahead and draw up a frame with square tubing. It will look like sh$t. Engineering aside, esthetics is a big part of the equation. People are drawn to pleasing curved lines.
Except for the success of Volvo...niteschaos
Jan 22, 2004 9:29 AM
I guess you have your finger on the pulse of bike buyers everywhere. Well, except on the pulse of this one.
Like I said, pleasing curved lines...spluti
Jan 22, 2004 10:13 AM
spluti, what volvo is that, I likey. nmcurtybirdychopper
Jan 22, 2004 10:45 AM
Like I said, pleasing curved lines...Juanmoretime
Jan 22, 2004 12:40 PM
More than engineering...weatherx
Jan 22, 2004 6:17 PM
like sh!t? imho not.
re: Why don't we see more bikes made with square tubes?MShaw
Jan 22, 2004 9:24 AM
There have been attempts by engineers to try this. All have failed miserably.

Kirk Magnesium frames come to mind immediately.

Some of it is tradition: bikes have always had round tubes... Some of it is looks. Some of it is "just because."

re: Why don't we see more bikes made with square tubes?upandcomer
Jan 22, 2004 2:24 PM
I think it would be a great idea to have an I beam for a seat tube where the greater majority of stresses are simple, out of plane bending, for which an I Beam is well suited. Then, put a fairing around it for increased aerodynamics and leave the other tubes round because they handle alot of twisting forces. The result would be a stiff light bike that isn't any less aero. It would be difficult to find a seatpost for it though....
How many more do you want?czardonic
Jan 22, 2004 5:49 PM
There are many bikes that have square tubing in the rear triangle -- granted, no road frames come to mind.
How many more do you want?Woof the dog
Jan 22, 2004 10:23 PM
K2 has oval top tube
Specialized s-works i think, Festina colored (not sloping tube), had an almost square top tube near the head tube.

just some info